The present invention relates to a device and a method for controlling the output of a rechargeable battery mounted on a vehicle.
A hybrid electric vehicle (HEV) using an engine and a motor as a power source has been put to practical use and is becoming popular. An HEV includes a rechargeable battery functioning as a power supply for supplying power to the motor.
When the engine outputs more power than necessary to drive the vehicle, the HEV drives its generator using the excessive power to charge the rechargeable battery. When the vehicle is braking or decelerating, the HEV drives the motor with the vehicle wheels and charges the rechargeable battery using the motor as a power generator. When the engine outputs less power than necessary, the HEV compensates for the lack of power by discharging the rechargeable battery and driving the motor.
In this way, the HEV accumulates energy in the rechargeable battery. Conventional automobiles release such energy into the atmosphere as heat. The energy efficiency of an HEV is higher than the energy efficiency of a conventional automobile. Thus, the HEV greatly improves fuel efficiency as compared with conventional automobiles.
An HEV supplies power to a starter motor, which is used to start the engine, with the rechargeable battery that supplies power to the drive motor. However, in the rechargeable battery, the discharge voltage or state of charge (SOC) greatly decreases at low temperatures of zero degrees Celsius or lower. The HEV may not be able to start the engine under such circumstances. Japanese Laid-Open Patent Publication No. 2002-195138 describes a controller (hereinafter referred to as “battery ECU”) for a rechargeable battery that limits the output of the discharged power to ensure starting of the engine under low temperatures.
Specifically, the battery ECU sets the minimum voltage of the rechargeable battery for driving the starter motor. If the discharge voltage that was greater than or equal to the minimum voltage when activating the starter motor becomes lowers than the minimum voltage, the battery ECU temporarily stops the supply of power. As discharge current generates heat in the battery and increases the discharge voltage, the battery ECU allows power to be supplied again to the starter motor. The battery ECU performs such a process before the engine starts. Therefore, in the above publication, the battery ECU ensures the starting of the engine under low temperatures by repeatedly stopping and starting discharge of the rechargeable battery.
The rechargeable battery mounted on the HEV is normally formed by connecting a plurality battery cells in series. If the capacity of each battery cell varies, the discharge capability of each battery cell also varies. As a result, excess discharge of a battery cell having a lowered capacity causes polarity inversion. Repeated starting and stopping of discharge within a short period of time also results in a tendency for polarity inversion to occur. Thus, in the invention of the above publication that repeatedly activates and inactivates the starter motor, the possibility of rise in the frequency of polarity inversion increases. This shortens the life span of the rechargeable battery.